1. Field of the Invention
The present invention relates to a method for preventing interference between radars and to a radar system having a function of preventing interference between radars.
2. Description of Related Art
A radar system for detecting a target (such as a preceding vehicle or an obstacle) by transmitting radar waves and receiving waves reflected by the target has been known hitherto. If plural radars are mounted on a vehicle, interference between radars has to be prevented to secure detecting accuracy of the radars.
When a pair of radars are mounted on a preceding vehicle M1 and a following vehicle M2, as shown in FIG. 5A, there is a possibility that interference occurs between a rear radar Rr of the preceding vehicle M1 and a front radar Rf of the following vehicle M2. Further, when a pair of radars Ra and Rb each detecting obstacles located in neighboring areas, as shown in FIG. 5B, radar waves transmitted by the radar Rb may be received by the radar Ra, causing interference between radars Ra and Rb.
To prevent interference between two radars, JP-A-2004-170183 proposes a radar system having a device for detecting the interference. When an interference with another radar is detected in the radar system, either one of a center frequency, a modulation method, a scanning width, a scanning direction, a scanning cycle period and scanning timing is changed in the radar system. In the proposed system, however, it is required to include additional devices for detecting and preventing the interference. This makes the system complex and expensive. In the case where two radars are mounted on the same vehicle, as shown in FIG. 5B, it is possible to prevent the interference by synchronizing operation timing between two radars.
However, it is difficult to apply such a counter measure to the inference between two radars mounted on different vehicles, as shown in FIG. 5A. If the rear radar Rr of the preceding vehicle M1 and the front radar Rf of the following vehicle M2 happen to be operated at a same time when both radars have a same cycle period, interference between both radars occurs continuously, as shown in FIG. 6A (in FIGS. 6A and 6B, either Rr or Rf is labeled R1 and the other is labeled R2). Even when the cycle periods of both radars are different from each other, as shown in FIG. 6B, the interference may occur continuously more than two times. When the interference occurs continuously more than two times, detecting accuracy of the radar system is greatly damaged.